1. Field of the Invention
This invention is concerned with slow release fertilizers and to processes for preparing same by encapsulating a sulfur-coated fertilizer with bitumen.
The rate of dissolution of fertilizers in the soil greatly exceeds the rate at which they are assimilated. Once they have dissolved, they are easily washed away by rain, resulting in considerable wastage. One way of reducing this wastage is to coat the fertilizer granules with a hydrophobic material. Encapsulation has now become a recognized way of making slow release fertilizers.
2. Description of the Prior Art
Encapsulated or coated fertilizers have been the subject of much research and a great many coating materials and methods have been patented in, for example, U.S. Pat. Nos: 3,223,518; 3,142,558; 3,248,255; 3,285,733; 3,190,741; 3,129,091; 3,220,824; 3,264,089; 3,119,683; 3,192,030; 3,198,761; 3,192,031 and 3,192,032.
As shown by these patents, various materials can be used for coating or encapsulating fertilizers. These include sulphur, paraffin wax, synthetic and natural resins, copolymers, bitumen and polyolefins. Sulphur has the advantage of providing additional nutritional benefits to the plants. Several techniques can be used for coating the fertilizers granules. The best technique appears to be spraying and, for this, special rotating drums have been built. Spraying of a molten material has the advantage, that when the film cools and so contracts, a tight skin is formed on the fertilizer granules which is more water-resistant. Powder coating has also been used. In this case the fertilizer to be coated is first heated to soften its surface without melting and is then contacted with a powdered coating agent.